Conventionally a rolling bearing unit comprising a pair of angular contact ball bearings 5 and 6 as shown in FIG. 6 is used in order to support a shaft 2 such as the rotating shaft of a screw compressor rotating at a high speed. The pair of ball bearings 5 and 6 are combined in a face-to-face arrangement.
It should be noted that if the pair of ball bearings 5 and 6 are combined in a back-to-back arrangement, due to the higher rigidity of the shaft with respect to the bending moment, the following characteristics to the bending moment are made worse, causing serious heat generation and seisure.
The bearing unit in the face-to-face arrangement in FIG. 6 is provided between an outer peripheral face of a rotating shaft 2 fixed to a rotor 1 of the screw compressor rotating at a high speed, and an inner peripheral face of a stationary housing 3. Furthermore the ball bearings 5 and 6 have an inner ring 10a and 10b and an outer ring 13a and 13b with balls 11 and 12 therebetween, respectively, and arranged to be used with a plus-clearance in the face-to-face arrangement, in other words to hold a clearance between the ball bearings 5 and 6 during a high speed rotation. It should be noted that if the ball bearings 5 and 6 are provided with a preload, which is sometimes referred to as a minus-clearance, therebetween, the inner load of the bearings becomes large, causing serious heat generation and seisure, so as not to be used for high rotation.
In FIG. 6, another rolling bearing 4 is provided with spacers 7 and 8 between the rolling bearing 4 and the bearing unit comprising the pair of ball bearings 5 and 6, and a retaining member 9 is mounted to the housing 3 at the axially outer side of the bearing unit.
The present invention is directed to an improvement of the duplex angular contact ball bearing unit used to support an axial load Fa on the rotating shaft 2. The term "axial" means the left and right directions of FIG. 6. On the other hand, the bearing 4 is provided for supporting a load Fr in a radial direction.
In this bearing unit, the ball bearings 5 and 6 juxtaposed in an axial direction have a pathway-or raceway 14a and 14b, respectively, which is made concave to bear the axial load on the rotating shaft 2. The direction of the contact angles .alpha.(alpha) of the ball bearings 5 and 6 (refer to FIG. 7) are in opposite directions to each other in the so-called face-to-face arrangement. Consequently, when there is a tendency for the rotating shaft 2 to be displaced to the left in FIG. 6, the left side ball bearing 5 in the same figure supports the axial load. Contrarily, when there is a tendency for the rotating shaft 2 to be displaced to the right, the right side ball bearing 6 supports the axial load. Hence the rotating shaft 2 and the rotor 1 are prevented from axially moving relative to the housing 3.
With the present invention, the situation is considered wherein a substantially unidirectional axial loading Fa (from right to left in the figure) is applied to the rotating shaft 2 rotating together with the rotor 1 during running.
With the duplex angular contact ball bearing unit comprising the pair of angular contact ball bearings 5 and 6 which are arranged in a fixed positioning relationship so as to be prevented from positionally changing with respect to each other, there is a problem that adequate bearing life cannot be achieved with high speed rotation of the shaft 2.
FIG. 7 illustrates the forces acting on the bearing unit as a result of the centrifugal forces produced by high speed rotation. More specifically, when the rotating shaft 2 rotates at a high speed, a centrifugal force Fc1 acts on the balls 11 and 12 of the respective ball bearings 5 and 6. Due to this centrifugal force, the balls 11 and 12 of the respective ball bearings 5 and 6 are pressed with a force Qin1 in a direction of the contact angle .alpha.(alpha) against the outer raceways or pathway 14a and 14b, respectively. It should be noted that the axial components of the forces Qinl on the ball bearings 5 and 6 are opposed to each other. Consequently, the balls 11 of the ball bearing 5 are pressed in an axially leftward direction against the outer raceway 14a under an axially leftward force component Fain1 of the force Qin1, while the balls 12 of the ball bearing 6 are pressed in an axially rightward direction against the outer raceway 14b under an axially rightward force component Fain1 of the force Qin1.
As a result, an internal axial loading corresponding to the sum (Fain1 +Fain1) of the components of force Qin1 is produced in the assembled ball bearings 5 and 6. Hence the pressure on contact surface between the balls 11 and 12 and the inner raceways 14a and 14b is increased, resulting in a reduction in fatigue life of the respective ball bearings 5 and 6.
To prevent a reduction in bearing life due to the above cause, the balls 11 and 12 in the both of the ball bearings 5 and 6 may be made from a light weight ceramic material, or balls of a small diameter may be used for both rows of the balls 11 and 12 to reduce the centrifugal force Fc1. Further, it is expected to make the contact angle smaller in the both rows of balls 11 and 12 to reduce Fain1.
However, when both rows of balls 11 and 12 are made from a ceramic material having a large modulus of direct elasticity, a pressure on contact surface between the outer raceways 14a and 14b and the balls 11 due to the balls 11 pressed against the outer raceways 14a and 14b is generally greater than the value for when the balls are made from bearing steel. Particularly with the ball bearing 5 which supports an external load (for example, the left side ball bearing 5 subjected to the beforementioned axial load Fa as a dotted arrow in FIG.7), the pressure is so large that the fatigue life of the ball bearing 5 is reduced, and the total fatigue life of the assembled ball bearings 5 and 6 is reduced.
Furthermore, when both rows of balls 11 and 12 have small diameter balls or small contact angle balls, the axial load capacity of the respective ball bearings 5 and 6 is reduced resulting in a reduction in fatigue life.
The angular contact ball bearing units of the present invention are addressed towards the above circumstances.